There are a number of ways you can get into the stock photography market as a seller. Some photographers decide to represent themselves, which means they handle all the aspects of marketing their own stock photos. This may take more time, but it will allow you more artistic control as well as put more money into your pocket.
Other stock photographers choose to market with only one representative as as an exclusive agreement. Those who pursue this option have the capability of creating a "brand image" from their name and a regular client list who likes to buy their work.
Still, other photographers allow some other entity to represent them. This means they will make less money per photo, but they will have more time to shoot photos and will spend less time with administrative work.
There are advantages and disadvantages to these choices. The most important thing is to make a conscious choice that is right for you so you don't regret your decision later on.